Why do Catholics need an annulment if they get a divorce?

Fr. Lous Sirianni explains why and when previously married Catholics may require an annulment. Divorced Catholics are not required to get an annulment in order to fully participate in the Catholic church and receive the sacraments. Unless their first marriage was declared invalid, the annulment process should be looked at when they seek a new marriage. They are still considered by the church to be in a valid union, even if they have gotten a divorce.
In the past, an individual might have been excommunicated if they attempted a second marriage in the civil form outside the church. This is no longer the case today, but the second marriage would be considered invalid, or an irregular union. Those considered in an irregular union are normally deprived of the seven sacraments.

From the Commonly Asked Questions section of The Spiritual Pilgrim website,